Thin-film elements formed on a flexible substrate such as a resin substrate, such as thin-film transistors or PIN-junction thin-film diodes, have widely been researched and developed to realize a flexible electronic device such as a flexible display, a flexible solar battery utilizing characteristics that it is light, has no crack, is flexible, and the like. Particularly, regarding the flexible display, an organic electroluminescence (EL) display which is more advantageous in flexibility has actively been researched and developed in comparison with a liquid crystal display requiring precise cell gap control.
In general, a flexible substrate such as a resin substrate has lower heat resistance than that of a glass substrate or the like. Accordingly, when a base film or thin-film elements are formed on the resin substrate or a barrier film is formed on the surface of the resin substrate opposite to the thin-film elements, a low-temperature process of about 200° C. or lower is necessary. Accordingly, the research and development has progressed with a focus on how to form a high-quality thin film or thin-film element at a low temperature.
As a thin-film element having a characteristic of relatively high quality in the low-temperature process, a thin-film transistor using oxide semiconductor for an active layer has attracted attention in recent years. Particularly, a flexible organic EL display using the oxide semiconductor thin-film transistor as a switching element of a pixel has actively been researched and developed.
Regarding flexible organic EL displays using an oxide semiconductor thin-film transistor on a resin substrate, papers such as IDW/AD'12 Proceedings of the 19th International Display Workshops, p. 851 and IDW/AD'12 Proceedings of the 19th International Display Workshops, p. 855 have been reported. In both papers, a polyimide layer as a resin substrate is formed on a glass substrate by application, oxide semiconductor thin-film transistors, an organic EL layer including a light-emitting layer, a carrier injecting and transporting layer, and electrodes, and a sealing layer are formed on a substrate with a monolithic structure of the polyimide layer and the glass substrate, and finally mechanically separating the glass substrate from the polyimide layer, thereby a flexible organic EL display is manufactured.
Formation of thin-film elements such as thin-film transistors directly on a resin substrate as a single body has a high wall against commercialization in view of difficulty in a low-temperature process, difficulty in precise positioning of thin-film patterns on the resin substrate, and the like. In order to solve this problem, a technique of separating high-performance and high-precision thin-film elements which have temporarily been formed on a glass substrate by high-temperature processes from the glass substrate using a certain method and the thin-film elements onto a resin substrate is disclosed.
For example, a quartz vibrator using single-crystal thin-film quartz which is obtained by forming an oxide single-crystal film containing GeO2 as a major component on a single-crystal substrate, forming the single-crystal thin-film quartz thereon, and dissolving GeO2 in an aqueous solution to separate them is disclosed (Japanese Patent Laid-Open No. 8-213871).
As a similar technique, a technique of forming a stacked structure of protective layer/thin-film element/GeOx/glass, separating the glass by immersing the stacked structure in water or the like and dissolving the GeOx, and forming a flexible thin-film device by transferring by bonding a support base (film or the like) to a separation surface and separating the protective layer is disclosed (Japanese Patent Laid-Open No. 2006-216891).
As a technique similar to the above-mentioned technique disclosed in the non-patent literature, a technique of forming a resin film on a glass substrate, forming thin-film elements thereon, and separating the glass substrate by irradiating the rear surface of the glass substrate with ultraviolet light is disclosed (Japanese Patent Laid-Open No. 2008-292608).
A technique of forming a protective film (a protective film having high blocking performance of gas which may give a certain damage to elements when a film is formed directly on the elements) having high gas barrier capability in advance on a heat-resistant substrate other than a substrate having elements formed thereon, transferring the protective film onto the substrate having elements formed thereon, and sealing the resultant structure is disclosed (Japanese Patent Laid-Open No. 2005-178363). Alternatively, a technique of coating one surface or both surfaces of a resin substrate with a diamond-like carbon (DLC) film as a protective film, bonding the resultant structure to a glass substrate with an adhesive layer interposed therebetween, forming elements thereon, and separating the glass substrate from the adhesive layer by irradiation with a laser beam is disclosed (Japanese Patent Laid-Open No. 2014-211638).
As described above, various techniques are disclosed as methods of manufacturing a flexible thin-film device or a flexible display device.
In such techniques, active elements such as thin-film transistors are first formed on a glass substrate on which a resin film (resin substrate) is formed by using application or coating, display elements such as organic EL elements, liquid crystal elements, or electrophoretic elements are formed thereon, and then the glass substrate is separated. In consideration of heat resistance of the resin substrate, it is necessary to form a passivation film of an inorganic insulating film or the like at about 300° C. at the time of formation of the active elements such as thin-film transistors. By forming a film at about 300° C., it is possible to form an insulating film having certain high barrier capability. In this way, a film having relatively high barrier capability can be formed on the active elements.
On the other hand, thin-film transistors are formed on the resin substrate through a process temperature of about 300° C. Accordingly, there is a problem in that adhesiveness between the glass substrate and the resin substrate increases due to the temperature and a yield decreases at the time of finally mechanically separating the glass substrate as described in the related art. No barrier film is formed on the rear surface of the resin substrate after the glass substrate is separated. In the techniques disclosed in the literatures, a barrier film can be formed on the rear surface of the resin substrate after the glass substrate is separated, but a low-temperature film formation technique at 100° C. or lower is necessary in consideration of heat-resistant temperatures of organic materials which are used in organic EL elements, liquid crystal elements, electrophoretic elements, or the like. The barrier capability of a film obtained by such a low-temperature film formation technique is very low.
When the barrier capability of the rear side of the resin substrate is low in this way, the following problems become marked. As disclosed in FIG. 4 of Non-patent Literature 1, a flexible device is manufactured by separating the glass substrate from the resin substrate after active elements (thin-film transistors in this case) or display elements (organic EL elements in this case) are formed. In this step, a barrier film is not formed on the rear surface of the resin film and thus the resin substrate starts moisture absorption immediately after the separation and thus swells more or less. This swelling causes formation of an unintended warp or fold in the flexible device, which is a severe problem in production. Accordingly, it is very important to secure high barrier capability on the rear surface side of the resin substrate. In order to effectively suppress the moisture absorption on the rear surface side of the resin substrate, high-temperature film formation techniques of 350° C., preferably, 400° C. or higher are required. On the contrary, when a barrier film is formed after the glass substrate is separated, organic EL elements are formed in advance, thus low-temperature film formation techniques of 100° C. or lower are required, and thus a film having low barrier capability is obtained. Alternatively, it is very difficult to form a barrier film on a substrate having organic EL elements formed thereon. As well as the organic EL elements, the same problem is caused when liquid crystal elements or electrophoretic elements as display devices are formed on thin-film transistors.
As described above, the following problems are apparent in the related art.
When the glass substrate is mechanically or physically separated from the resin substrate after a thin-film device including active elements and display elements is formed, a separation yield decreases due to an influence of an adhesive force between the resin substrate and the glass substrate. For example, when a region partially having a stronger adhesive force is present at random, the region is not separated well, which causes a decrease in yield.
It is very difficult to perform a process of forming a barrier film on the rear surface of the resin substrate after separation. For example, when a barrier film is formed on the rear surface of the resin substrate of a thin-film device in which thin-film transistors and display elements are stacked on the resin substrate, it is difficult to carry the thin-film device in process equipment and it is necessary to perform considerable low-temperature film formation of about 100° C. or lower. The barrier capability of a thin film formed by this low-temperature film formation is low.
All the techniques in the related art have common problems in that separation yield is low and barrier capability of a barrier film is low in a structure having the barrier film against infiltration of gas or moisture on the rear surface of the resin substrate.